A residual image, which is a common defect of Liquid Crystal Displays (LCDs) nowadays, refers to a phenomenon that, when a single picture has been displayed on a LCD for a long time and suddenly changed to another picture, the display content of the last picture can be indistinctly visible. This is because, during manufacture of a LCD, due to presence of various factors, there may be some ions in a liquid crystal box, as shown in FIG. 2(a). While displaying, these ions move in direction of positive and negative electrical fields (i.e., driving electrical fields). When the movement energy of the ions applied by the positive and negative electrical fields is not uniform, these ions will accumulate on one side gradually. When the accumulated ions reach a certain concentration, the transmission rate of the panel will be changed. When the display is suddenly changed to the next picture, as shown in FIG. 2(b), some of the accumulated ions cannot be scattered immediately and will maintain the original picture, resulting in a residual image.
Conventionally, a residual image can typically be tested as follows. A 3*3 grid picture as shown in FIG. 1 is displayed for a long time (typically about 1 hour) and then changed to a gray scale picture and, in this case, a residual image phenomenon will occur. An image residual level of the display can be evaluated based on the time length required for the residual image to disappear. The time length required for the residual image to disappear is typically obtained by optically observing a surface luminance of the panel, and then modified based on the evaluation. There are problems that the residual image level does not improve or degrade obviously, the evaluation is not accurate enough and it is difficult for accurate modification.